1. Field of the Invention
This invention relates to polycarbonates and more particularly to polycarbonate blends having an improved impact performance.
2. Description of the Prior Art
Polycarbonates derived from reactions involving organic dihydroxy compounds and carbonic acid derivatives have found extensive commercial application because of their excellent mechanical and physical properties. These thermoplastic polymers are particularly suited for the manufacture of molded article products for which impact strength, rigidity, toughness, thermal and dimensional stability as well as excellent electrical properties are required.
However, one deficiency of polycarbonate when used in molded articles is the low critical thickness values of polycarbonate polymer, which deficiency tends to limit wall thickness to a value below the critical thickness.
It is known that polycarbonate plastics exhibit high notched Izod (ASTM test D-256) impact values. This value, however, is dependent upon the thickness of the test specimen. Typical notched Izod impact values for a one-eighth inch specimen are about 16 ft.-lbs. per inch. These high Izod values result because specimens of one-eighth inch thickness are thinner than the critical thickness of the polymer and are associated with a hinged or a ductile break. On the other hand, one-fourth inch specimens exhibit a clean or brittle break having notched Izod impact values of only about 2.5 ft.-lbs. per inch. The one-fourth inch specimens are said to be above the critical thickness of the polymer. "Critical thickness" has been defined as the thickness at which a discontinuity in the graphical representation of Izod impact strength vs. specimen thickness occurs. This thickness, in fact, indicates a transition from ductile to brittle performance.
Thus, a standard impact specimen of polycarbonate polymer thicker than the critical thickness exhibits brittle breaks and those thinner than the critical thickness exhibit hinged or ductile breaks. Further, the critical thickness of a polycarbonate based on bisphenol A with a melt flow of 3 to 6 grams/10 minutes at 300.degree. C. (ASTM D1238) has a critical thickness of about 225 mils. The critical thickness of polycarbonates have been shown (see U.S. Pat. No. 3,437,631) to improve upon the incorporation of polyolefins. Other approaches entail the incorporation of sulfur containing aromatic diols into the polycarbonate chain (see U.S. Pat. No. 4,043,980).
The preparation and use of copolymers of olefins and maleic anhydride have been disclosed in U.S. Pat. Nos. 3,637,579; 3,598,778; 3,729,450; 3,560,456; 3,586,659; 3,732,337 and 3,637,904.
U.S. Pat. No. 3,862,998 discloses a polycarbonate composition comprising a polycarbonate resin and a styrene maleic anhydride copolymer characterized by its improved hydrolytic stability.
U.S. Pat. No. 4,172,859 teaches, inter alia, tough polycarbonate compositions comprising random copolymers of maleic anhydride and an unsaturated monomer having pendant hydrocarbon chains of 1 to 12 carbon atoms.